Is your feature request related to a problem? Please describe.
Currently we cannot insert newlines consistently, nor can we go to previous messages.

Describe the solution you'd like
I wanna be able to insert newlines and go to previous messages.

Describe alternatives you've considered
vim

Additional context

We could revamp it so that the popup has two parts, like the custom command popup.
The first one would just for the summary of the commit, with options such as:

• pressing Up or Down goes to the previous or next commit message, respectively
• pressing Enter would confirm the commit (just like it does now)
• pressing Tab would go to the part below

The lower one would be for the description of the commit with the following features:

• pressing Enter would insert a newline
• pressing Up or Down would move the cursor Up or Down
• pressing Tab would switch back to the upper textbox

The discussion is here.